Theoretical Study of the Low-Lying Electronically Excited States of OBrO

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1 J. Phys. Chem. A 2003, 107, Theoreticl Study of the Low-Lying Electroniclly Excited Sttes of OBrO Reinhrd Vetter,* Thoms Ritschel, nd Lutz Zu1licke Institut für Chemie, UniVersität Potsdm, Postfch , D Potsdm, Germny Kirk A. Peterson Deprtment of Chemistry, Wshington Stte UniVersity, Pullmn, Wshington nd The Willim R. Wiley EnVironmentl Moleculr Sciences Lbortory, Pcific Northwest Ntionl Lbortory, Richlnd, Wshington ReceiVed: August 23, 2002 Motivted by the possible importnce of OBrO in tmospheric photochemistry, multireference configurtion interction clcultions of the low-lying excited sttes were crried out to obtin informtion bout the electronic verticl spectrum up to excittion energies of bout 6 ev from the ground stte, including the trnsition dipole moments, nd bout possible photodissocition pthwys, bsed on one-dimensionl cuts through the potentil energy surfces for dissocition into BrO + O nd Br + O 2, respectively. In ddition, for probing the ngle dependence the bending potentils were lso clculted. From ll computed eight doublet sttes (two/four of ech symmetry in C 2V C s ) only the 1 2 A 2 stte t 2.7 ev possesses lrge trnsition dipole moment with the 1 2 B 1 ground stte, wheres for ll other sttes this quntity is very smll or zero. Therefore the 1 2 A 2 stte should ply decisive role in OBrO photochemistry. Close to the 1 2 A 2 stte two other sttes were found t 2.4 ev (1 2 B 2 ) nd 2.5 ev (1 2 A 1 ) so tht interctions of these three sttes should certinly influence possible dissocition processes. For this reson, besides direct dibtic photodissocition of the 1 2 A 2 stte into BrO + O lso predissocition vi these close-lying sttes cn be expected, leding to very complex photodissocition mechnism for excittion energies round 2.5 ev. Moreover, in this energy rnge photodissocition into Br + O 2 is only possible through the 1 2 B 2 stte (fter initil excittion of the 1 2 A 2 stte) becuse only for this stte smll brrier of 0.7 ev reltive to its minimum is estimted from the clcultion of simplified C 2V minimum energy pth. For the 1 2 A 1 nd 1 2 A 2 sttes, rther lrge brriers re predicted. The next higher-lying sttes, with excittion energies of 3.9 ev (2 2 A 1 ) nd 4.5 ev (2 2 B 2 ) re well seprted from lower- nd higher-lying sttes nd from ech other, but due to their smll trnsition dipole moments, they should be probbly of minor importnce for the OBrO photochemistry. The lst two sttes considered in our study re predicted to lie close together t 6.0 ev (2 2 A 2 ) nd 6.1 ev (2 2 B 1 ) nd re strongly repulsive upon dissocition into BrO + O. Finlly, it should be noted tht our clcultions demonstrte the expected qulittive similrity to the results previously obtined for the corresponding OClO system. Introduction It hs become highly probble in the pst decde tht in ddition to chlorine, bromine oxides (especilly BrO) lso ply n importnt role in ctlytic cycles of strtospheric ozone destruction. This fct stimulted the detiled experimentl nd theoreticl investigtion of the properties nd processes involving these species. 1 Up to the present time, the knowledge bout bromine oxides is fr from being complete, s cn be clerly seen by comprison of the hlogen dioxides OClO nd OBrO. Contrry to OClO which is well chrcterized experimentlly nd theoreticlly, 2-5 much less is known bout OBrO. 5,6 This is not unexpected, becuse the investigtion of the OBrO rdicl turns out to be more difficult experimentlly (very unstble system) s well s theoreticlly (electron-rich system). Moreover, the prticiption of OBrO in ozone destruction processes is presently still not quite cler, becuse spectroscopic observtions in the strtosphere nd model clcultions gve contrdictory results. Wheres blloon-borne spectroscopic investigtions of Renrd et l. 7 presumbly detected OBrO in the strtosphere in significnt quntities, model clcultions performed by * Corresponding uthor. E-mil: vetter@tcb12.chem.uni-potsdm.de. Chipperfield et l. 8 indicte much smller bundnces of this species. Recently, mesurements of OBrO limits in the nighttime strtosphere by Erle et l. 9 suggest negligible role of OBrO in strtospheric photochemistry s well. Thus, the importnce of OBrO in tmospheric bromine chemistry remins uncertin nd more informtion bout the photochemistry of OBrO, especilly photodissocition cross sections, is highly desirble. At present, only few experimentl nd theoreticl studies deling with the electroniclly excited sttes of OBrO re vilble, minly limited to the chrcteriztion of the 1 2 A 2 (C 2 A 2 ) stte. The first visible spectrum of OBrO in the gs phse due to the C 2 A 2 r X 2 B 1 electronic trnsition ws reported by Rttign et l. 10 Somewht lter, Miller et l. 11 reinvestigted the spectrum nd mde use of the results of quntum-chemicl clcultions for relible interprettion of the experimentl findings. Furthermore, two studies of the electronic spectrum in rre gs mtrices were lso crried out. 12,13 Importnt for comprison with theoreticl predictions is the first mesurement of UV/vis bsorption cross sections for gs-phse OBrO t room temperture by Knight et l. 14 Finlly, two theoreticl ppers deling with electroniclly excited sttes of OBrO should be mentioned. In the first pper, /jp021952p CCC: $ Americn Chemicl Society Published on Web 02/08/2003

2 1406 J. Phys. Chem. A, Vol. 107, No. 9, 2003 Vetter et l. Miller et l. 11 crried out CCSD(T) clcultions to determine the geometries nd reltive energies of the four lowest-lying sttes X 2 B 1, A 2 B 2, B 2 A 1, nd C 2 A 2. In the second pper, Peterson 5 used the multireference configurtion interction method to clculte ner-equilibrium potentil energy functions (PEF) for the X 2 B 1 nd C 2 A 2 sttes. On the bsis of the PEF results the vibrtionl spectr were clculted. Altogether, there is still reltively scrce informtion on this interesting nd importnt system; becuse of tht we strted more comprehensive clcultions of the low-lying excited sttes of OBrO with the finl im of predicting cross sections for the relevnt photodissocition processes. In first step, we report in this pper bout extensive CASSCF-MRCI clcultions of the electronic verticl spectrum (eight doublet sttes nd four qurtet sttes) nd the corresponding trnsition dipole moments from the X 2 B 1 ground stte. To get first impression bout possible photodissocition pthwys one-dimensionl cuts through the full potentil energy surfces for dissocition into BrO + O s well s Br + O 2 were clculted. Finlly, bending potentils re lso presented. Computtionl Detils All clcultions were crried out t the multireference internlly contrcted configurtion interction (icmrci) level of theory using the MOLPRO b initio pckge. 15 The MRCI clcultions were bsed on complete ctive spce self-consistent field (CASSCF) orbitls. 16,17 The ctive spce in the CASSCF consisted of the nine orbitls rising from the 2p nd 4p tomic orbitls of oxygen nd bromine, respectively (13 ctive electrons in 9 orbitls). All other low-lying orbitls were fully optimized, but constrined to be doubly occupied. The clcultions of the electronic spectrum, the bending potentils, nd the potentil curves for dissocition into Br + O 2 were crried out in C 2V symmetry, while the tretment of dissocition into BrO + O ws in C s. The size of the resulting CAS ws 473 CSFs (configurtion stte functions) for the B 1 sttes, 477 CSFs for the A 1 nd B 2 sttes, nd 463 CSFs for the A 2 sttes. In C s symmetry the corresponding numbers re 954(A ) nd 936(A ). The reference function for the subsequent MRCI clcultions employed the ctive spce s in the CASSCF but extended by the doubly occupied two oxygen 2s nd the bromine 4s orbitls. This mens ll 19 vlence electrons were correlted. All single nd double excittions with respect to this reference function were included in the MRCI nd the doubly externl configurtions were internlly contrcted 18,19 (icmrci). Energy contributions for higher-order excittions hve been estimted by the multireference nlog 20,21 of the Dvidson correction 22 (icmrci+q). For oxygen nd bromine, the ug-cc-pvtz bsis sets of Dunning nd co-workers were employed with omission of the diffuse f functions. The totl bsis set thus obtined is denoted s AVTZ ; it consisted of the following contrcted functions: [4s3p2d1f] for ech oxygen nd [6s5p3d1f] for bromine with one diffuse s, p, nd d function on ech tom. To estimte the influence of the bsis set flexibility on the excittion energies of OBrO, severl other bsis sets were pplied nd re compiled in Tble 1. The electronic ground stte of OBrO in C 2V symmetry is of 2 B 1 type nd corresponds t its equilibrium geometry to the electronic configurtion Core (9 1 ) 2 (5b 2 ) 2 (10 1 ) 2 (11 1 ) 2 (6b 2 ) 2 (4b 1 ) 2 (7b 2 ) 2 (2 2 ) 2 (12 1 ) 2 (5b 1 ) 1 TABLE 1: Summry of Bsis Sets Employed bsis set (bbrevition) cgtos or Dunning s nottion comments VTZ 103 cc-pvtz b,d AVTZ 151 ug-cc-pvtz b,c,d AVTZ 130 ug-cc-pvtz without diffuse f functions AVTZ (ECP) 116 ug-cc-pvtz optimized for using in combintion with qusireltivistic effective core potentil (ECP) e nd neglecting the diffuse f functions AVTZ +sp 142 ug-cc-pvtz ugmented by n dditionl set of diffuse s nd p functions f but without diffuse f functions AVQZ 205 ug-cc-pvqz b,c,d without diffuse f nd g functions Number of bsis functions (contrcted Gussin Type Orbitls (cgtos)). b Ref 23. c Ref 24. d Ref 25. e Ref 5. f The exponents of the dditionl set of diffuse s nd p functions were determined in n eventempered sense s proposed in ref 26: oxygen s ( ) nd p ( ); bromine s ( ) nd p ( ). TABLE 2: Verticl Electronic Excittion Energies E, Trnsition Dipole Moments R e e, nd Oscilltor Strengths f of the Lowest-Lying Doublet Sttes of OBrO stte (C 2V C s) dominnt configurtions E (ev) R e e (u) b,c 1 2 B 1/1 2 A... (7b 2) 2 (2 2) 2 (12 1) 2 (5b 1) B 2/1 2 A... (7b 2) 1 (2 2) 2 (12 1) 2 (5b 1) A 1/2 2 A... (7b 2) 2 (2 2) 2 (12 1) 1 (5b 1) (y) A 2/2 2 A... (7b 2) 2 (2 2) 1 (12 1) 2 (5b 1) (x) A 1/3 2 A... (7b 2) 2 (2 2) 2 (12 1) 2 (5b 1) 0 (13 1) (y) B 2/4 2 A... (7b 2) 2 (2 2) 2 (12 1) 2 (5b 1) 0 (8b 2) A 2/3 2 A... (7b 2) 1 (2 2) 2 (12 1) 2 (5b 1) 1 (13 1) (x) B 1/4 2 A... (7b 2) 2 (2 2) 2 (12 1) 1 (5b 1) 1 (13 1) (z) 0.0 icmrci+q results, AVTZ bsis nd experimentl (C 2V) geometry used (ref 27). b In prentheses: polriztion of the trnsition (OBrO is plced in the xz plne with z being the C 2 xis for C 2V symmetry). c The 2 B 2 r 2 B 1 trnsitions re electric dipole forbidden. Core (13 ) 2 (14 ) 2 (15 ) 2 (16 ) 2 (17 ) 2 (5 ) 2 (18 ) 2 (6 ) 2 (19 ) 2 (7 ) 1 in C s symmetry. It should be mentioned tht the ground-stte occuption given in the pper of Miller et l. 11 Core (14 1 ) 2 (2b 2 ) 2 (3 2 ) 2 (7b 1 ) 1 ppers to be in error. The electronic verticl spectrum of OBrO hs been clculted t the experimentl equilibrium geometry of Müller et l. 27 (Br-O distnce Å ( bohr); <) OBrO ). Results nd Discussion The Verticl nd Adibtic Electronic Excittion Energies. As first step towrd discussing photodissocition, it is useful to get n overview of the relevnt excited electronic sttes by clculting the electronic verticl spectrum. This informtion is especilly importnt for OBrO, becuse up to now only one excited stte hs been experimentlly investigted, which ws chrcterized by visible bsorption spectrum in the cm -1 ( nm) region with extensive vibronic structure nd n intensity mximum t bout cm -1 (496 nm) Only the three lowest-lying excited sttes were theoreticlly studied by Miller et l. 11 nd Peterson. 5 We hve therefore extended the clcultions to include totl of two sttes for ech C 2V point group symmetry. Moreover, some preliminry three-stte clcultions were lso crried out. The clculted verticl excittion energies of doublet sttes re presented in Tble 2, together with the trnsition dipole moments nd oscilltor strengths for excittions from the ground f

3 Low-Lying Electroniclly Excited Sttes of OBrO J. Phys. Chem. A, Vol. 107, No. 9, stte. From Tble 2 it follows tht the three lowest-lying verticl excittion energies re obtined close together between 2.4 nd 2.7 ev (i.e., 517 nd 459 nm), in greement with the findings of Miller et l. 11 for the corresponding dibtic trnsitions, which will be discussed somewht lter in more detil. These three sttes possess 2 B 2, 2 A 1, nd 2 A 2 symmetry nd result from excittions of the 7b 2,12 1, nd 2 2 orbitls into the singly occupied 5b 1 orbitl. Another four electronic sttes re predicted to lie in the energy rnge up to bout 6 ev which is still interesting for tmospheric photochemistry. Those t 3.9 ev (2 2 A 1 ) nd 4.5 ev (2 2 B 2 ) re creted by excittions from the singly occupied orbitl 5b 1 into the lowest-lying virtul orbitls 13 1 nd 8b 2. The lst two sttes (2 2 A 2 nd 2 2 B 1 ) considered in our two-root clcultions show excittion energies round 6 ev. Preliminry three-root clcultions predict four further sttes between 6 nd 7 ev (6.0 ev (3 2 B 2 ), 6.4 ev (3 2 B 1 ), 6.5 ev (3 2 A 1 ), nd 6.7 ev (3 2 A 2 )), deriving from 2 2 f 13 1,12 1 f 13 1,11 1 f 5b 1, nd 7b 1 f 13 1 trnsitions. The clculted high density of sttes bove 6 ev will mke experimentl nd theoreticl investigtion of possible photodissocition processes in this energy region much more difficult. For n ssignment of vilble experimentl electronic spectr nd for first impression of those sttes which could be importnt for photodissocition we must lso look t the electronic trnsition moments or the oscilltor strengths, which re relted to the intensities of the trnsitions from the ground stte. Tble 2 clerly shows tht the most importnt excited stte will be the 1 2 A 2 stte. This is the only stte with lrge trnsition dipole moment (0.594 u), wheres for ll other sttes with exception of the second 2 A 2 stte this quntity is very smll (exctly zero for the electric dipole forbidden trnsitions from the ground stte to the 2 B 2 sttes). From this it follows tht the strongest experimentlly observed bsorption feture in the UV spectrum of OBrO t bout 2.5 ev (496 nm) origintes from the 1 2 A 2 (C 2 A 2 ) r 1 2 B 1 (X 2 B 1 ) trnsition nd photodissocition processes in this energy region should strt with n excittion to the 1 2 A 2 stte. A comprison with clcultions of the corresponding chlorine system, OClO, is only possible for the three lowest excittions nd shows fetures similr to OBrO: three close-lying sttes of 2 B 2, 2 A 1, nd 2 A 2 symmetry, but with 0.7 ev to 1 ev higher trnsition energies ( 3.2 ev (1 2 B 2,1 2 A 1 ) estimted from Figure 5 of ref 28 nd 3.66 ev (1 2 A 2 ) 28 ). Before treting lrger prts of the potentil energy surfces of OBrO, it is importnt to estimte the influence of bsis set flexibility, especilly the role of diffuse functions on the clculted verticl excittion energies. The bsis sets considered hve been derived from the triple-ζ correltion consistent bsis sets of Dunning nd co-workers nd denoted s VTZ in this pper. Moreover, one bsis set of qudruple-ζ qulity ws used (cf. Tble 1). The results of this bsis set comprison re compiled in Tble 3. For studying the influence of diffuse functions, the VTZ bsis ws ugmented with one set of diffuse s, p, d, nd f functions (AVTZ). In somewht more economicl vrint, the diffuse f functions of the AVTZ bsis were neglected (AVTZ ). Compring the clculted VTZ, AVTZ, nd AVTZ excittion energies, only minor chnges were observed by dditionl inclusion of diffuse functions (the differences mounted only up to 0.04 ev). This is lso true when dding second set of diffuse s nd p functions (AVTZ + sp). Therefore ll sttes considered would pper to possess no mrked Rydberg chrcter. A further increse of bsis set flexibility by chnging TABLE 3: Verticl Electronic Excittion Energies E for the Doublet Sttes of OBrO. Influence of Bsis Set Flexibility nd Role of Diffuse Functions E (ev) bsis set VTZ AVTZ AVTZ AVTZ +sp AVQZ AVTZ (ECP) stte 1 2 B B A A A B A B icmrci+q results, experimentl geometry used (ref 27). to bsis set of qudruple-ζ qulity (AVQZ ) seems to be not necessry s well, s cn be seen from the results of Tble 3. Summrizing ll the bsis set investigtions we cn conclude tht the AVTZ bsis set is good compromise between bsis set qulity nd economy; therefore it will be used in ll subsequent clcultions. Finlly, the excittion energies were lso clculted using slightly modified AVTZ bsis set nd reltivistic effective core potentil (ECP) for bromine. 5 The AVTZ (ECP) results presented in Tble 3 show tht the verticl excittion energies were not significntly ffected by this type of ECP pproximtion; the differences to the AVTZ excittion energies re only 0.1 ev t most. After discussion of the verticl excittion energies we turn now to the dibtic ones. The following clcultions should minly be considered s further test of the efficiency nd relibility of the method (icmrci+q) nd bsis set (AVTZ ) employed. This test is possible by comprison with the CCSD(T) clcultions of Miller et l. 11 (TZ2P bsis) nd the MRCI study of Peterson 5 with somewht lrger bsis sets of qudruple-ζ qulity, nd moreover by comprison with experimentl findings (only for the 1 2 B 1 nd 1 2 A 2 sttes). As cn be seen from Tble 4, our theoreticl predictions of the geometries nd excittion energies of the four lowest-lying sttes of OBrO re in very close greement with the previous, bove-mentioned theoreticl clcultions. The differences of the Br-O bond length nd OBrO bond ngle re t most 0.01 Å nd 0.8, respectively, nd in the cse of the excittion energies t most 0.06 ev. The differences to the experimentl vlues for the 1 2 B 1 nd 1 2 A 2 sttes re even smller. As lredy pointed out for the verticl excittion energies, there re similrities with the corresponding chlorine system (Tble 4, lst column). In both systems we observe n extension of the X-O bond length (X ) Cl, Br) upon excittion for ll three sttes ( 2 B 2, 2 A 2, 2 A 1 ), wheres the bond ngle for the 1 2 A 2 stte decreses nd for the 1 2 A 1 stte increses slightly. It should be emphsized tht the 1 2 B 2 stte of both OBrO nd OClO is n cute-ngle stte. The differences in the dibtic excittion energies of the bromine nd chlorine systems re found to be in the rnge 0.4 ev to 0.6 ev, somewht smller thn those clculted for the verticl excittion energies. At the end of this section we present some results of the clcultion of the low-lying qurtet sttes of OBrO, since excittions from the doublet ground stte into qurtet sttes vi spin-orbit coupling should be possible. The first four qurtet sttes were found to lie close together between 5 nd 6 ev (Tble 5). Photodissocition processes linked to the lowest-lying excited sttes of OBrO, especilly to

4 1408 J. Phys. Chem. A, Vol. 107, No. 9, 2003 Vetter et l. TABLE 4: Geometries nd Adibtic Electronic Excittion Energies T eb for the Lowest Bound Doublet Sttes of OBrO. Comprison to OClO OBrO (X ) Br) OClO (X ) Cl) stte this work e (icmrci + Q) KAP f (icmrci + Q) MNFS g CCSD(T) KAP/PW h (icmrci + Q) 1 2 B 1 r(xo) c <) (OXO) c B 2 r(xo) <) (OXO) T e A 2 r(xo) c r(oxo) c T d e A 1 r(xo) <) (OXO) T e Bond lengths in Å, bond ngles in degrees. b T e in ev. c Experimentl vlues (OBrO): Å nd (1 2 B 1) (ref 27) nd ( Å nd ( 0.5 (1 2 A 2) (ref 11). d Experimentl vlues: 1.99 ev (OBrO, ref 11), 2.01 ev (OBrO, ref 5), nd 2.68 ev (OClO, ref 5). e AVTZ bsis used. f Ref 5. g Ref 11. h Ref 5 for the 1 2 B 1 nd 1 2 A 2 stte. Ref 28 for the others. TABLE 5: Verticl Electronic Excittion Energies E for the Lowest-Lying Qurtet Sttes of OBrO b stte dominnt configurtions E (ev) 1 4 B 2... (7b 2) 2 (2 2) 1 (12 1) 2 (5b 1) 1 (13 1) A 2... (7b 2) 1 (2 2) 2 (12 1) 2 (5b 1) 1 (13 1) B 1... (7b 2) 2 (2 2) 2 (12 1) 1 (5b 1) 1 (13 1) A 1... (7b 2) 2 (2 2) 1 (12 1) 2 (5b 1) 1 (8b 2) Relted to the 1 2 B 1 ground stte of OBrO. b icmrci+q results (AVTZ bsis), experimentl geometry used (ref 27). Figure 1. Correltion digrm for the dissocition of OBrO into BrO + O nd Br + O 2, ssuming C s symmetry for the BrO + O nd C 2V symmetry for the Br + O 2 chnnel. Full lines represent A symmetry in C s, dshed lines correspond to A symmetry. The numbers refer to the clculted electronic dissocition energies (without spin-orbit contributions); in prentheses: experimentl estimtions (cf. Tble 6). the 1 2 A 2 stte, should not be influenced by qurtet sttes, but for excittion energies lrger thn 5 ev processes involving qurtet sttes cnnot be excluded. Interesting to note, the qurtet sttes of the corresponding chlorine dioxide system were previously clculted to be locted t considerbly higher energies (6.82 ev ( 4 B 2 ), 7.96 ev ( 4 A 2 ), 8.08 ev ( 4 A 1 ), nd 8.27 ev ( 4 B 1 )). 28 Correltion Digrm for Dissocition into BrO + O nd Br + O 2. Before strting with ctul clcultions of some cuts through the potentil energy surfces of OBrO, it is useful for qulittive understnding of the dissocition processes to drw correltion digrm for dissocition into both possible chnnels BrO + O nd Br + O 2 (Figure 1). The verticl excittion energies of OBrO in Figure 1 re tken from Tble 2. The (electronic) dissocition energies were TABLE 6: Comprison of Clculted Dissocition Energies (supermolecule pproch ) nd Experimentl Estimtions electronic dissocition energies (ev) b experimentl dissocition chnnel icmrci+q (AVTZ ) estimtions c BrO(X 2 Π) + O( 3 P) d BrO(X 2 Π) + O( 1 D) e Br( 2 P o ) + O 2( 3 Σ ḡ ) f Br( 2 P o ) + O 2( 1 g) g Br( 2 P o ) + O 2 ( 1 Σ + g ) 1.23 h (BrO + O): Br-O bohr Br-O bohr (exp. vlue for BrO 29 ) <) OBrO (exp. vlue for OBrO 27 ). (Br + O 2): Br- O 1(2) 10.0 bohr O-O 2.30 bohr ( 3 Σ ḡ ) nd 2.32 bohr ( 1 g)(exp. vlues: 2.28 bohr nd 2.30 bohr 30 ). b All vlues relted to the OBrO(1 2 B 1) ground stte. c To mke vlid comprisons with the theoreticl predictions, zero-point vibrtionl energy (ZPE) nd spinorbit (SO) contributions re removed from the experimentl estimtes. d Estimted by tking the hets of formtion fh o 0 (OBrO: 1.80 ev; 6 BrO: 1.38 ev; 31 O: 2.56 ev 6 ) nd corrected for ZPE (OBrO: ev; 11 BrO: ev 30 ) s well s SO contributions (BrO: 0.06 ev; 30 O: 0.01 ev 32 ). e Estimted from the experimentl energy difference O( 1 D) - O( 3 P): 1.97 ev. 32 f Estimted by tking the hets of formtion fh o 0 (OBrO: 1.80 ev; 6 Br: 1.22 ev; 6 O 2: 0 ev (stndrd stte)) nd corrected for ZPE (OBrO: ev; 11 O 2: ev 30 ) s well s SO contributions (Br: 0.15 ev 32 ). g Estimted from the experimentl energy difference O 2( 1 g) - O 2( 3 Σ ḡ ): 0.98 ev 30 (T e). h Estimted from the experimentl energy difference O 2( 1 Σ + g ) - O 2( 3 Σ ḡ ): 1.64 ev 30 (T e). clculted in supermolecule pproch nd re compred with estimted ( experimentl ) vlues obtined from experimentl dt for hets of formtion nd experimentl excittion energies of the oxygen tom nd the oxygen molecule, nd corrected for spin-orbit nd zero-point energy contributions. Detils cn be found in Tble 6. To the uthors knowledge experimentlly determined dissocition energies re not vilble up to now. The devitions of bout 0.2 ev between theoreticl predictions nd experimentl findings re expected for this level of theory nd bsis set. In estblishing the correltions, we ssumed C s symmetry for the BrO + O dissocition chnnel nd C 2V for the Br + O 2 chnnel; only doublet sttes of OBrO hve been tken into ccount. The BrO(X 2 Π) ground-stte splits into n A nd n A stte in C s symmetry. The interction with the O( 3 P) tom (one A nd two A components in C s ) produces three sttes of A nd three of A symmetry. First, from Figure 1 it follows tht for these six sttes the 1 2 A (1 2 B 1 ) ground stte of OBrO nd the

5 Low-Lying Electroniclly Excited Sttes of OBrO J. Phys. Chem. A, Vol. 107, No. 9, three close-lying excited sttes 1 2 A (1 2 B 2 ), 2 2 A (1 2 A 1 ), nd 2 2 A (1 2 A 2 ) re vilble for correltion. Furthermore, lso the 3 2 A (2 2 A 1 ) nd 3 2 A (2 2 A 2 ) sttes correlte with BrO nd O in their ground sttes. Hence photodissocition processes with excittion energies of bout 2.5 ev, which re highly probble becuse of the lrge trnsition dipole moment between the 1 2 B 1 ground nd 1 2 A 2 excited stte (Tble 2), should produce bromine oxide nd the oxygen tom in their electronic ground sttes. The interction of BrO in its ground stte with oxygen in its first excited 1 D stte gives 10 doublet sttes, five of A nd five of A symmetry, but only one stte of ech symmetry (4 2 A (2 2 B 2 ) nd 4 2 A (2 2 B 1 )) ws clculted in this work. As lredy mentioned in the lst section, besides the 1 2 A 2 stte only the second 2 A 2 stte possesses trnsition moment pprecibly lrger thn zero (Tble 2) nd therefore lso for higher excittion energies of bout 6 ev the dissocition products (BrO, O) should be formed exclusively in their ground sttes. Consider now the right side of Figure 1, describing the dissocition into Br + O 2.InC 2V symmetry the Br( 2 P ) groundstte splits into three components of 2 A 1, 2 B 1, nd 2 B 2 symmetry, wheres the ground stte of O 2 (X 3 ḡ ) becomes 3 B 1 stte. The interction of these sttes of Br nd O 2 produces doublet nd qurtet sttes of symmetry B 1,A 1, nd A 2 ech; the doublets correlte with the three lowest 1 2 B 1,1 2 A 1, nd 1 2 A 2 sttes of OBrO. It is interesting to note from the correltion digrm tht the 1 2 B 1 (1 2 A ) ground stte of OBrO is unstble with respect to dissocition to the ground-stte products Br( 2 P ) nd O 2 (X 3 ḡ ), which lie 0.5 ev below the 1 2 B 1 stte of OBrO. Becuse OBrO hs been experimentlly detected nd investigted (cf. the Introduction), brrier in the dissocition chnnel must exist to prevent dissocition into Br + O 2 (see lso the next section). The first excited stte of O 2 hs 1 g symmetry (splitting into 1 A 1 nd 1 B 1 in C 2V ) nd couples with Br( 2 P ) to give two A 1, two B 1, one B 2, nd one A 2 stte, but only four sttes were clculted in the present work (2 2 A 1,2 2 B 1,1 2 B 2, nd 2 2 A 2 ) nd re shown in Figure 1. The interction of Br( 2 P ) with O 2 in its excited 1 g + stte leds to three sttes of the types 2 A 1, 2 B 1, nd 2 B 2, but only one of them, the 2 2 B 2 stte, ws considered in our clcultions. Finlly, it follows from the correltion digrm in Figure 1 tht upon breking C 2V symmetry to C s the crossing between the 1 2 B 2 nd the 1 2 A 1 sttes chnges to n voided one, becuse both become 2 A sttes. This will certinly influence the dissocition dynmics of OBrO into Br + O 2. Potentil Energy Curves for the BrO + O nd Br + O 2 Dissocition Pthwys. For detiled quntittive theoreticl description of the dissocition dynmics, complete threedimensionl potentil energy surfces of the relevnt electronic sttes of OBrO re indispensble nd clcultions long these lines re plnned for the ner future. However, for the moment, to get first feeling bout the behvior of the excited sttes of OBrO upon dissocition, some one-dimensionl cuts through the globl surfces were clculted. Figure 2 shows the potentil energy curves of the four lowestlying doublet sttes of A nd A symmetry, respectively, s functions of one Br-O bond length, keeping the other Br-O distnce nd the bond ngle fixed t the experimentl groundstte equilibrium vlues. Importnt to note, but not unexpected, the shpes of the curves for the two lowest-lying sttes of A nd A symmetry upon dissocition re very similr to results obtined previously for OClO by Peterson nd Werner (cf. Figure 6 of ref 28). Contrry to the strongly bound ground stte (1 2 B 1 ) with dissocition energy ( E diss ) of 2.1 ev, the first Figure 2. Clculted potentil energy curves of the eight lowest-lying doublet sttes of OBrO for symmetric dissocition into BrO + O, keeping one Br-O distnce nd the OBrO bond ngle fixed t the experimentl ground-stte vlues. Full lines represent sttes of A, dshed lines of A symmetry. excited 1 2 A (1 2 B 2 ) stte is only very wekly bound ( E diss 0.3 ev). In the cse of OClO it ws found tht this stte is unbound. 28 The next two close-lying sttes with 2 2 A (1 2 A 1 ) nd 2 2 A (1 2 A 2 ) symmetries re both wekly repulsive in the outer prt nd smll brrier of bout 0.5 ev ppers long the bond stretching coordinte in nlogy to the OClO system. 28 The brriers t Br-O distnce of bout 3.75 bohr (2 2 A ) nd bout 4.25 bohr (2 2 A ) re the result of voided crossings with the next higher 3 2 A (2 2 A 1 ) nd 3 2 A (2 2 A 2 ) sttes, s cn be seen in Figure 2. But especilly interesting is the fct tht in the Frnck-Condon region ll three sttes (1 2 B 2,1 2 A 1,1 2 A 2 ) re found close together in the energy rnge 2.4 ev-2.7 ev nd therefore very complex photodissocition mechnism should be expected. Certinly direct dibtic dissocition fter excittion of the 2 2 A (1 2 A 2 ) stte is possible, becuse of the lrge trnsition dipole moment clculted for the 2 2 A r 1 2 A trnsition (Tble 2), nd provided tht the excittion energy is lrge enough ( E g 2.8 ev) to overcome the smll brrier. Contrry to this, direct dibtic dissocition through the 1 2 A (1 2 B 2 ) nd 2 2 A (1 2 A 1 ) sttes is not probble becuse the 1 2 B 2 r 1 2 B 1 trnsition is dipole forbidden nd the 1 2 A 1 r 1 2 B 1 trnsition possesses only very smll trnsition moment, s lredy mentioned in the preceding section. For the indirect dissocition (predissocition) of the 2 2 A (1 2 A 2 ) stte in the cse of OClO, Peterson nd Werner 28 hve discussed three-step mechnism which should lso be vlid for the bromine system becuse of the similrity of the potentils. According to this mechnism the 2 2 A (1 2 A 2 ) stte intercts with the 2 2 A (1 2 A 1 ) stte vi spin-orbit coupling followed by dissocition of the 2 2 A (1 2 A 1 ) stte nd/or by vibronic coupling of the 2 2 A (1 2 A 1 ) nd 1 2 A (1 2 B 2 ) sttes nd frgmenttion on the 1 2 A (1 2 B 2 ) surfce. The next two well-seprted excited sttes with clculted verticl excittion energies of 3.9 nd 4.5 ev hve A symmetry nd dissocite into two different chnnels producing oxygen in the O( 3 P) ground stte nd in the first excited O( 1 D) stte, respectively (Figure 2). Both sttes re more or less strongly repulsive nd possess smll brrier long the O-BrO dissocition coordinte. However, these two sttes should be of minor importnce for discussing photodissocition of OBrO becuse the trnsition dipole moments for the 2 2 A 1 r 1 2 B 1 nd the 2 2 B 2 r 1 2 B 1 trnsitions re very smll or zero (Tble 2). Somewht more interesting re the two close-lying 2 A (2 2 A 2, 2 2 B 1 ) sttes with excittion energies of bout 6 ev. Both sttes re strongly repulsive upon dissocition; they produce, in nlogy to the two A sttes discussed before, oxygen in the

6 1410 J. Phys. Chem. A, Vol. 107, No. 9, 2003 Vetter et l. Figure 3. Clculted potentil energy curves of the four lowest-lying doublet sttes of OBrO for symmetric (C 2V) dissocition into Br + O 2; the OBrO bond ngle ws optimized for ech clculted Br-O distnce. For those sttes which would become of A (A ) type upon breking C 2V symmetry, full (dshed) lines re used. ground stte or in the first excited stte, respectively, nd show nondibtic interctions in the Frnck-Condon region. But lso for these two sttes the trnsition moments re very smll (Tble 2) so tht efficient photodissocition for excittion energies of 6 ev should not be observed. We turn now to the other possible dissocition chnnel for OBrO leding to Br + O 2 (Figure 3). For these clcultions we hve ssumed simplified minimum energy pth under the constrint of C 2V symmetry. The potentil energy curves for the four lowest-lying sttes of OBrO (X 2 B 1,1 2 B 2,1 2 A 1, nd 1 2 A 2 ) hve been computed s functions of the symmetriclly stretched Br-O bond lengths; the OBrO bond ngle ws optimized for ech considered Br-O distnce. From Figure 3 it follows tht, with exception of the 1 2 B 2 stte, the other three sttes studied hve reltively lrge brriers E B tht must be overcome for dissocition leding to Br nd O 2 in their ground sttes ( E B pproximtely equl to 4.0 ev (1 2 B 1 ), 2.0 ev (1 2 A 1 ), nd 1.8 ev (1 2 A 2 ), reltive to the respective potentil mimimum, corresponding to threshold energies of 4.0, 4.2, nd 3.9 ev, respectively, reltive to the ground-stte minimum. Becuse of the C 2V symmetry constrint the brrier heights re upper limits, but the results of Figure 3 clerly show tht the 1 2 B 2 stte should ply decisive role for the dissocition dynmics of the Br + O 2 chnnel since the brrier for dissocition is considerbly smller in comprison to those of the sttes discussed bove ( E B ) 0.7 ev reltive to the 1 2 B 2 minimum nd 2.3 ev reltive to the 1 2 B 1 groundstte minimum). For the sme resons s for the BrO + O chnnel, direct excittion nd dibtic dissocition is improbble, but the 1 2 B 2 stte should be ccessible from initil excittion nd predissocition of the 1 2 A 2 stte. A further peculirity of the 1 2 B 2 stte compred with the other three sttes investigted is the dditionl shllow minimum in the dissocition chnnel ( E B only 0.4 ev reltive to this minimum) which could be symmetric configurtion of the isomeric BrOO system, s discussed by Peterson nd Werner 4 for the corresponding chlorine system. It cn be certinly ssumed tht this smll brrier will not decisively influence the dissocition process. Figure 3 shows tht the dissocition of the 1 2 B 2 stte produces the oxygen molecule in the first excited stte, but it should be emphsized tht for symmetric geometries (C s symmetry) n voided crossing between the 1 2 A 1 nd the 1 2 B 2 sttes tkes plce lte in the exit chnnel, since both sttes re then of 2 A symmetry. Therefore it cn be expected tht the O 2 product is Figure 4. Clculted potentil energy curves of the eight lowest-lying doublet sttes of OBrO s functions of the OBrO bond ngle R, keeping both Br-O distnces fixed t their experimentl ground-stte vlues. Full nd dshed lines re used s in Figure 3. formed not only in the first excited electronic stte but lso in the ground stte. Finlly, it should be mentioned tht the findings for OBrO just discussed re in qulittive greement with results obtined for OClO in somewht more detiled two-dimensionl tretment. 4 The brriers E B for OClO re found to be only slightly higher (t the most 0.6 ev) in comprison to the corresponding OBrO system, nmely 4.6 ev (X 2 B 1 ), 2.2 ev (1 2 A 1 ), 1.8 ev (1 2 A 2 ), nd 1.0 ev (1 2 B 2 ) reltive to the respective potentil minimum, nd threshold energies of 4.6, 4.8, 4.4, nd 2.8 ev, respectively, reltive to the ground-stte minimum. Potentil Energy Curves for the Bending Motion. Besides one-dimensionl cuts for describing the dissocition into BrO + O nd Br + O 2, it should be useful for better understnding of the photochemistry of OBrO to probe the ngle dependence of ll eight sttes considered (Figure 4). As cn be expected from the results discussed in the lst section, the bending potentils in Figure 4, chrcterized by severl curve crossings, give further indiction of the rther complicted shpes of the excited-stte potentil energy surfces of OBrO, even for the three lowest-lying sttes with verticl excittion energies round 2.5 ev. Figure 4 shows tht the eight sttes form four Renner-Teller pirs with 2 Π u, 2 g, 2 Π g, nd gin 2 Π g symmetry, respectively, t the liner configurtion. In nlogy to the potentil curves for dissocition (cf. the lst section), lso the bending behvior of the 1 2 A 1,1 2 A 2,1 2 B 1, nd 1 2 B 2 potentils shows qulittive greement with the corresponding OClO potentils (cf. Figure 4 of ref 28). Looking t Figure 4 in more detil, we observe two importnt crossings in the Frnck-Condon region. The first one between the 1 2 A 2 nd the 1 2 A 1 stte is even llowed in C s symmetry nd, s discussed by Peterson nd Werner for the OClO system, 28 possible predissocition mechnism of the 1 2 A 2 stte should be medited by the interction of the 1 2 A 2 nd 1 2 A 1 stte through spin-orbit coupling. The second crossing between the 1 2 B 2 nd 1 2 A 1 stte is chnged to n voided crossing upon C s distortions, so tht in ddition to the 1 2 A 1 stte, lso the 1 2 B 2 stte is possibly involved in the predissocition of the 1 2 A 2 stte vi vibronic coupling. It should be further noted tht the 1 2 A 1 stte hs only very flt minimum in the Frnck-Condon region nd the liner configurtion possesses lower energy thn the bent form. The smll brrier between bent nd liner

7 Low-Lying Electroniclly Excited Sttes of OBrO J. Phys. Chem. A, Vol. 107, No. 9, Figure 5. ()-(c) Clculted trnsition dipole moments (x nd z components) connecting the three excited 2 A sttes with the 1 2 A ground stte s functions of one Br-O bond length, keeping the other Br-O distnce nd the bond ngle fixed t the experimentl ground-stte vlues. OBrO is plced in the xz plne with the origin of the coordinte system t the center of mss nd the xes coinciding with the principl xes of the inerti tensor. structure is the result of n voided crossing of the 1 2 A 1 nd 2 2 A 1 sttes t n OBrO ngle of bout 135. Two further crossings in C 2V, which become voided ones in C s symmetry, re observed between the 1 2 B 2 nd 2 2 A 1 sttes t bout 127 nd between the 2 2 A 1 nd 2 2 B 2 sttes t bond ngle of bout 110. Finlly it should be mentioned tht the higher-lying sttes (2 2 B 1,2 2 A 2, nd 2 2 B 2 ) possess two minim in the bending coordinte, with exception of the 2 2 A 1 stte (<) OBrO 135 ). These minim re found t very smll ngles on one hnd nd t ngles t or ner the liner configurtion on the other. The brriers between the two minim re probbly the result of voided crossings with higher-lying sttes, not clculted in this work. Geometry Dependence of the Trnsition Dipole Moments. For quntittive theoreticl description of photodissocition dynmics besides the relevnt potentil energy surfces, lso the trnsition dipole moments, connecting the ground nd excited sttes hve to be clculted s functions of the nucler (internl) coordintes. Therefore these quntities will be shortly discussed in the following, tking into ccount tht for excittions from the vibrtionl ground stte the trnsition dipole moment functions ner to the Frnck-Condon region ply the decisive role. In Figures 5-c, the trnsition dipole moments connecting the three clculted excited 2 A sttes with the 1 2 A ground stte re presented s functions of one Br-O bond length. The corresponding vlues for the excited sttes of 2 A symmetry remin very smll or zero under oxygen tom bstrction s lredy found for the verticl trnsitions in the equilibrium ground-stte geometry (Tble 2). This implies tht direct photoinduced dissocition of these sttes is very improbble. Figures 5-c show completely different behvior of the trnsition dipole moment functions for trnsitions to the 2 2 A stte on one hnd nd to the 3 2 A nd 4 2 A sttes, respectively, on the other. The x component of the trnsition dipole moment to the 2 2 A stte (Figure 5) is lrge in the Frnck-Condon region (bout 0.6 u), emphsizing the importnce of this stte for photodissocition processes of OBrO. At lrger Br-O distnces, we observe steep decrese to zero. The z component is zero for the symmetric C 2V geometry, remins considerbly smller thn x nd lso pproches zero for lrger distnces. Contrry to this, both components of the trnsition dipole moments to the next two higher-lying 2 A sttes re very smll in the Frnck-Condon region nd become zero for bromine tom bstrction (Figures 5b nd 5c). Therefore, direct photodissocition through these sttes should not be very probble, s lredy found for the sttes with A symmetry. The vritions of the trnsition dipole moments t Br-O distnce of bout 3 u nd especilly in the rnge of u re cused by voided crossings tking plce in these regions (cf. Figure 2). Conclusions The results nd discussions presented in the preceding sections show tht for photodissocition processes of bromine dioxide rther complex behvior is to be expected. The density of sttes nd the resulting coupling of electronic terms in the energy rnge up to 6 ev will mke OBrO interesting for tmospheric photochemistry. Furthermore, the findings of our study demonstrte the expected similrity to the corresponding chlorine system. Efficient (direct) photodissocition into BrO + O should tke plce fter excittion of the 1 2 A 2 /2 2 A stte t n excittion energy of 2.7 ev becuse of the lrge trnsition dipole moment clculted for the 2 2 A /1 2 A 2 r 1 2 A /1 2 B 1 trnsition. But it should be mentioned tht for the elucidtion of the role of the smll brrier in the dissocition chnnel of the 2 2 A stte, three-dimensionl tretment - clcultion of the complete potentil energy surfce - is necessry. Importnt to note is tht in nlogy to the OClO system in the Frnck-Condon region, two further sttes (1 2 B 2 /1 2 A nd 1 2 A 1 /2 2 A ) re predicted close to the 1 2 A 2 /2 2 A stte. Though both sttes possess only smll (or zero) trnsition dipole moments to the ground stte, predissocition should be possible vi nondibtic interctions with the 2 2 A stte. Clerly this complex sitution t excittion energies round 2.5 ev mkes relistic theoreticl description of the photodissocition dynmics of OBrO rther difficult. The next two sttes t 3.9 ev (2 2 A 1 /3 2 A ) nd 4.5 ev (2 2 B 2 / 4 2 A ) re well-seprted from lower- nd higher-lying sttes, but they should be of minor importnce for OBrO photodisso-

8 1412 J. Phys. Chem. A, Vol. 107, No. 9, 2003 Vetter et l. cition becuse of their very smll trnsition dipole moments to the ground stte. Further, our clcultions predict two closelying excited 2 A sttes t bout 6 ev. Both sttes re strongly repulsive upon dissocition into BrO + O producing oxygen in the ground or the first excited stte nd should exhibit nondibtic interctions in the Frnck-Condon region. Concerning dissocition into Br + O 2, the similrity to the corresponding OClO system is lso evident. The clcultions were crried out only for the four lowest-lying sttes (1 2 B 1,1 2 B 2, 1 2 A 1, nd 1 2 A 2 ) under the constrint of simplified minimum energy pth ssuming C 2V symmetry. Becuse of the lrge brriers which must be overcome for dissocition into Br + O 2 for the 1 2 B 1,1 2 A 1, nd 1 2 A 2 sttes (4.0, 2.0, nd 1.8 ev reltive to the respective potentil minimum), only the 1 2 B 2 stte with considerbly lower brrier of 0.7 ev should ply decisive role for the Br + O 2 chnnel. However the 1 2 B 2 r 1 2 B 1 trnsitions re dipole forbidden so tht direct excittion nd dissocition of the 1 2 B 2 stte should be improbble; but, s lredy discussed for the BrO + O chnnel, indirect dissocition fter excittion of the 1 2 A 2 stte is conceivble. For the intended extension of our investigtions towrd tretment of the OBrO photodissocition dynmics, complete three-dimensionl potentil energy surfces for the 1 2 A ground stte nd the most importnt 2 2 A excited stte re to be clculted; moreover the couplings with close-lying excited sttes must be included. Work in this direction is under wy. References nd Notes (1) Wyne, R. P.; Poulet, G.; Biggs, P.; Burrows, J. P.; Cox, R. A.; Crutzen, P. J.; Hymn, G. D.; Jenkin, M. E.; Le Brs, G.; Moortgt, G. K.; Pltt, U.; Schindler, R. N. Atmos. EnViron. 1995, 29, (2) Snder, S. P.; Friedl, R. R.; Frncisco, J. S. In Progress nd Problems in Atmospheric Chemistry; Brker, J. R. Ed.; World Scientific: Singpore, 1995; p 876. (3) Vid, V.; Simon, J. D. Science 1995, 268, (4) Peterson, K. A.; Werner, H.-J. J. Chem. Phys. 1996, 105, 9823, nd references therein. (5) Peterson, K. A. J. Chem. Phys. 1998, 109, 8864, nd references therein. (6) Klemm, R. B.; Thorn, R. P., Jr.; Stief, L. J.; Buckley, Th. J.; Johnson, R. D., III. J. Phys. Chem. A 2001, 105, 1638, nd references therein. (7) () Renrd, J.-B.; Pirre; M.; Robert, C.; Huguenin, D. J. Geophys. Res. 1998, 103, (b) Berthet, G.; Renrd, J.-B.; Chrtier, M.; Pirre, M.; Robert, C. J. Geophys. Res. Accepted for publiction (privte communiction by J.-B. Renrd). (8) Chipperfield, M. P.; Glssup, T.; Pundt, I.; Rttign, O. V. Geophys. Res. Lett. 1998, 25, (9) Erle, F.; Pltt, U.; Pfeilsticker, K. Geophys. Res. Lett. 2000, 27, (10) Rttign, O. V.; Jones, R. L.; Cox, R. A. Chem. Phys. Lett. 1994, 230, 121. (11) Miller, C. E.; Nickolisen, S. L.; Frncisco, J. S.; Snder, S. P. J. Chem. Phys. 1997, 107, (12) Kölm, J.; Engdhl, A.; Schrems, O.; Nelnder, B. Chem. Phys. 1997, 214, 313. (13) Lee, Y.-C.; Lee, Y.-P. J. Phys. Chem. A 2000, 104, (14) Knight, G.; Rvishnkr, A. R.; Burkholder, J. B. J. Phys. Chem. A 2000, 104, (15) MOLPRO (Version 96.4) is pckge of b initio progrms written by H.-J. Werner nd P. J. Knowles with contributions of J. Almlöf,R.D. Amos, A. Berning, M. J. O. Deegn, F. Eckert, S. T. Elbert, C. Hmpel, R. Lindh, W. Meyer, A. Nicklss, K. Peterson, R. Pitzer, A. J. Stone, P. R. Tylor, M. E. Mur, P. Puly, M. Schuetz, H. Stoll, T. Thorsteinsson, nd D. L. Cooper. (16) Werner, H.-J.; Knowles, P. J. J. Chem. Phys. 1985, 82, (17) Knowles, P. J.; Werner, H.-J. Chem. Phys. Lett. 1985, 115, 259. (18) Werner, H.-J.; Knowles, P. J. J. Chem. Phys. 1988, 89, (19) Knowles, P. J.; Werner, H.-J. Chem. Phys. Lett. 1988, 145, 514. (20) Blomberg, M. R. A.; Siegbhn, P. E. M. J. Chem. Phys. 1983, 78, (21) Simons, J. J. Phys. Chem. 1989, 93, 626. (22) Lnghoff, S. R.; Dvidson, E. R. Int. J. Quntum Chem. 1974, 8, 61. (23) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, (24) Kendll, R. A.; Dunning, T. H., Jr.; Hrrison, R. J. J. Chem. Phys. 1992, 96, (25) Wilson, A. K.; Woon, D. E.; Peterson, K. A.; Dunning, T. H., Jr. J. Chem. Phys. 1999, 110, (26) Lee, T. J.; Schefer, H. F., III. J. Chem. Phys. 1985, 83, (27) Müller, H. S. P.; Miller, C. E.; Cohen, E. A. J. Chem. Phys. 1997, 107, (28) Peterson, K. A.; Werner, H.-J. J. Chem. Phys. 1992, 96, (29) Alcmi, M.; Cooper, I. L. J. Chem. Phys. 1998, 108, (30) Huber, K. P.; Herzberg, G. Moleculr Spectr nd Moleculr Structure, IV. Constnts of Ditomic Molecules; Von Nostrnd Reinhold: New York, (31) Chse, M. W. J. Phys. Chem. Ref. Dt 1996, 25, (32) Moore, C. E. Atomic Energy LeVels, NSRDS-NBS 35, Office of Stndrd Reference Dt, Ntionl Bureu of Stndrds, Wshington, DC, 1971.